The present invention relates to a gas sensor installed in an exhaust gas system of an internal combustion engine for a combustion control or else. Furthermore, the present invention relates to a method for manufacturing this gas sensor.
According to a conventional gas sensor, a sensing element is inserted into an insulator. The insulator is fixed in a housing. A measured gas side cover is attached to a distal end of the housing. An air side cover is attached to a proximal end of the housing. The clearance between the insulator and the housing is airtightly sealed. Similarly, the clearance between the sensing element and the insulator is airtightly sealed.
Presence of such an airtight sealing makes it possible to separate an inside space of the gas sensor into an aerial atmosphere and a measured gas atmosphere.
In general, the sensing element has a measured gas sensing electrode exposed to a measured gas and a reference gas sensing electrode exposed to the air serving as a reference gas. The sensing element produces a sensing signal representing a gas concentration in the measured gas based on an ion current or an electric potential produced between these electrodes. Hence, measurement of gas concentration cannot be performed accurately when separation between the aerial atmosphere and the measured gas atmosphere is insufficient.
Conventionally, powdered material, such as talc, and a sealing glass are layered between the sensing element and the insulator to airtightly separate the aerial atmosphere and the measured gas atmosphere.
For example, U.S. Pat. No. 5,602,325 discloses a plurality of solid-phase sintered glass layers and a plurality of steatite spacer layers which are alternately stacked in a ceramic sensor holder. A ceramic main body surrounds the alternately stacked glass layers and spacer layers. The ceramic main body extends to an outside housing. Furthermore, thin solid-phase sintered glass layers are interposed between the ceramic main body and each spacer layer.
Furthermore, U.S. Pat. Nos. 5,467,636 and 5,739,414 disclose a glass layer interposed between a first ceramic insulating body and a second ceramic insulating body. According to this prior art, the glass is subjected to a compressive stress acting in the radial direction (i.e., in the central direction) within an operating temperature zone.
However, securing airtightness by filling the powdered material, such as talc, requires checking many items to administrate the pressure and the filling amount of the powdered material. This in disadvantageous in costs.
Furthermore, the glass layer is generally formed through the processes of placing the powdered glass material to a predetermined position, heating the powdered glass material to melt it, and then cooling the molten glass until it is hardened. This makes it difficult to obtain a highly densified glass sealing material. Accordingly, it is difficult to maintain satisfactory airtightness for a gas sensor based on a sealing arrangement using the glass sealing material only.
In view of the above-described conventional problems, an object of the present invention is to provide a gas sensor having an arrangement capable of sealing the clearance between the insulator and the sensing element with the glass material only. Furthermore, the present invention provides a manufacturing method for this sensor.
In order to accomplish the above and other related objects, the present invention provides a gas sensor comprising a cylindrical insulator, a sensing element airtightly fixed in the insulator, and a cylindrical housing having an inside space for placing the insulator, with an air side cover attached to a proximal end of this housing so as to confine an aerial atmosphere therein, wherein a glass sealing material seals a clearance between an inner surface of the insulator and an outer surface of the sensing element, and a proximal end surface of the glass sealing material protrudes toward a proximal end of the gas sensor at a contact interface of the glass sealing material to the inner surface of the insulator and to the outer surface of the sensing element compared with at least an adjacent portion of the remainder.
The present invention is characterized in that the glass sealing material seals a clearance between the inner surface of the insulator and the outer surface of the sensing element. The proximal end surface of the glass sealing material protrudes toward the proximal end of the gas sensor at the contact interface of the glass sealing material to the inner surface of the insulator and to the outer surface of the sensing element compared with at least an adjacent portion of the remainder.
Next, function of the present invention will be explained.
According to the present invention, the proximal end surface of the glass sealing material protrudes toward the proximal end of the gas sensor at the contact interface of the glass sealing material to the inner surface of the insulator and to the outer surface of the sensing element compared with at least an adjacent portion of the remainder (refer to FIG. 2). This arrangement makes it possible to firmly fix the glass sealing material to the sensing element and to the insulator at the contact interface thereof, thereby maintaining improved airtightness.
Accordingly, it becomes possible to surely provide an airtight sealing for the clearance between the sensing element and the insulator by using a single glass sealing material such as a glass pellet, i.e., without using powdered material, and without requiring multistage filling processes of the sealing material, and further without requiring complicated check of numerous managing items.
According to the present invention, it becomes possible to provide a gas sensor capable of sealing the clearance between the insulator and the sensing element with the glass material only.
The glass sealing material is, for example, a material whose composition is expressed by B2O3xe2x80x94ZnOxe2x80x94SiO2xe2x80x94Al2O3xe2x80x94BaOxe2x80x94MgO.
This material has an excellent sealing ability for the sensing element and the insulator. Thus, it becomes possible to ensure the reliable airtight sealing between the glass sealing material and the sensing element as well as between the glass sealing material and the insulator.
Furthermore, the present invention is applicable to a gas sensor incorporating a cup-shaped solid electrolytic sensing element as shown in FIG. 1, and also applicable to a gas sensor incorporating a multilayered sensing element.
Furthermore, the arrangement of the present invention is applicable to an oxygen sensor and to an air-fuel ratio sensor for an automotive internal combustion engine. Especially, when formed into a multilayered type, the arrangement of the present invention is preferably applicable to a NOx sensor, a CO sensor or the like.
Next, according to the present invention, it is preferable that a protruding portion of the proximal end surface corresponds to at least 98% of the contact interface which extends circumferentially along an entire periphery of the glass sealing material.
The expression xe2x80x9cat least 98% of the contact interfacexe2x80x9d means that the contact interface between the glass sealing material and the inner surface of the insulator and the contact interface between the glass sealing material and the outer surface of the sensing element protrude by an amount of 98% or more in the circumferential direction.
When the protruding portion exceeds 98%, it becomes possible to surely provide an airtight sealing for the clearance between the sensing element and the insulator by using a single glass sealing material.
If the protruding portion is less than 98%, gas leakage may occur.
Needless to say, it is most preferable that the proximal end surface of the glass sealing material protrudes toward the proximal end of the gas sensor entirely along the circumferentially extending contact interface.
Next, the present invention provides a method for manufacturing a gas sensor comprising a cylindrical insulator, a sensing element airtightly fixed in the insulator, and a cylindrical housing having an inside space for placing the insulator, with an air side cover attached to a proximal end of the housing so as to confine an aerial atmosphere therein and a measured gas side cover attached to a distal end of the housing so as to confine a measured gas atmosphere therein, wherein a glass sealing material seals a clearance between an inner surface of the insulator and an outer surface of the sensing element, and a proximal end surface of the glass sealing material protrudes toward a proximal end of the gas sensor at a contact interface of the glass sealing material to the inner surface of the insulator and to the outer surface of the sensing element compared with at least an adjacent portion of the remainder.
The method of the present invention comprises the steps of preparing a cylindrical glass pellet having an outer shape fitting to the inner surface of the insulator and having a through-hole into which the sensing element is inserted, inserting the glass pellet into the insulator and placing the sensing element in the through-hole of the glass pellet, and melting the glass pellet and then hardening the molten glass to firmly seal the clearance between the inner surface of the insulator and the outer surface of the sensing element.
According to the manufacturing method of the present invention, the glass pellet configured into a predetermined shape is inserted into the insulator. Then, the sensing element is disposed in the through-hole of the glass pellet. Thereafter, the glass pellet is melted and hardened to firmly seal the clearance between the insulator and the sensing element.
Accordingly, compared with a conventional method for directly filling the clearance with powdered glass material etc. as a glass sealing material, it becomes possible to realize a highly densified glass sealing. Accordingly, it becomes easy to obtain a desired sealing in length as well as in volume, thereby realizing a reliable sealing.
Accordingly, it becomes possible to firmly fix the sensing element and the insulator at their interfaces so as to maintain excellent airtightness. Furthermore, it becomes possible to surely provide an airtight sealing for the clearance between the sensing element and the insulator by using a single glass sealing material only.
As described above, the present invention makes it possible to provide a manufacturing method for a gas sensor capable of sealing the clearance between the insulator and the sensing element with the glass material only.
Regarding the shape of the glass pellet, it is possible to form the glass pellet with side surfaces fitting to the inner surface of the insulator and to the outer surface of the sensing element. It is also possible to configure the glass pellet to have the through-hole beforehand so that the sensing element can be inserted into this through-hole.
It is also possible to use the glass pellet consisting of a plurality of parts.